Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. In addition to the well-known photosensitive, so-called pre-sensitized plates, which are suitable for UV contact exposure through a film mask, also heat-sensitive printing plate precursors have become very popular in the late 1990s. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by crosslinking of a polymer, heat-induced solubilization or particle coagulation of a thermoplastic polymer latex.
The most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically comprises an oleophilic binder, e.g. a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support. Typical examples of such plates are described in e.g. EP-A 625728, 823327, 825927, 864420, 894622 and 901902. Negative working embodiments of such thermal materials often require a pre-heat step between exposure and development as described in e.g. EP-625,728.
Negative working plate precursors which do not require a pre-heat step may contain an image-recording layer that works by heat-induced particle coalescence of a thermoplastic polymer particle (latex), as described in e.g. EP-As 770 494, 770 495, 770 496 and 770 497. These patents disclose a method for making a lithographic printing plate comprising the steps of (1) image-wise exposing an imaging element comprising hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder and a compound capable of converting light into heat, (2) and developing the image-wise exposed element by applying fountain and/or ink.
Some of these thermal processes enable platemaking without wet processing and are for example based on ablation of one or more layers of the coating. At the exposed areas the surface of an underlying layer is revealed which has a different affinity towards ink or fountain than the surface of the unexposed coating.
Other thermal processes which enable platemaking without wet processing are for example processes based on a heat-induced hydrophilic/oleophilic conversion of one or more layers of the coating so that at exposed areas a different affinity towards ink or fountain is created than at the surface of the unexposed coating.
U.S. Pat. No. 5,030,550 discloses a developer for positive photoresists which contains a basic compound and a specific non-ionic surfactant comprising an oxyethylene and an oxypropylene group. For accelerating the dissolution of the photoresist compositions in a developer, polyhydroxy compounds such as novolac may be added to the developer.
JP 3062034 discloses a developer for positive photoresists which contains an organomettalic surfactant and an alkali soluble resin such as novolac.
US 2005/0175942 discloses a developer for printing plates comprising a polyoxyalkylene adduct of alkylene diamine, a polyoxyalkylene compound having an acid radical and an anionic surfactant.
A major problem associated with positive-working printing plate precursors based on a solubility difference is the low differentiation between the development kinetics of exposed and non-exposed areas—i.e. the dissolution of the exposed areas in the developer is not completely finished before the unexposed areas also start dissolving in the developer. This often results in insufficient clean-out which may become apparent as a reduction of the sensitivity of the plate. Reduced clean-out usually results in toning (ink-acceptance in the non-image areas) of the printing plate and/or in ink build-up on the blanket. The low differentiation between the development kinetics of exposed and non-exposed areas may further lead to a loss of coating in the image areas, especially a loss of small image details or so-called high lights; a reduced press life and/or a reduced chemical resistance. Basically, a too small difference in dissolving rate in the developer between the exposed and the unexposed areas results in virtually no processing latitude. In order to increase this difference in dissolving rate, a so-called image dissolution inhibiting agent, which is capable of preventing etching of the image-areas during developing, is often added to the developer solution. However, such compounds may interact with the image forming material which dissolves from the non-image areas into the developer during processing possibly resulting in insoluble matter in the processing bath, also referred to as sludge. This development sludge can be deposited on the printing plate which impairs the images formed thereon and/or can precipitate in the processing bath making the maintenance of the processing bath more burdensome. Furthermore, as such interactions consume the image dissolution inhibiting agent, the activity of the development solution changes within time. Indeed, it is often observed that during consecutive development steps, the development solution becomes more aggressive resulting in a higher solid/imaging material loss and/or a higher loss of fine image details. This phenomenon is known in the art as aggressiveness increase of the developer.